Prime DS

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karthikpandian19: Legendary Member; Posts: 1665; Joined: Thu Nov 03, 2011 7:04 pmFollowed by: 70 members; Thanked: 165 times

by karthikpandian19 » Thu Dec 15, 2011 7:13 pm

If y â‰ 3 and 3x/y is a prime integer greater than 2, which of the following must be true?
â… . x = y
â…¡. y = 1
â…¢. x and y are prime integers.

(A) None
(B) â… only
(C) â…¡only
(D) â…¢only
(E) â… and â…¢

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Source: Beat The GMAT — Problem Solving | Thu Dec 15, 2011 7:13 pm

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Thu Dec 15, 2011 8:38 pm

karthikpandian19 wrote:If y â‰ 3 and 3x/y is a prime integer greater than 2, which of the following must be true?
â… . x = y
â…¡. y = 1
â…¢. x and y are prime integers.

(A) None
(B) â… only
(C) â…¡only
(D) â…¢only
(E) â… and â…¢

(I) x = y
If x = 5, y = 5, then 3x/y = 3 (a prime) greater than 2.
If x = 25, y = 15, then 3x/y = 5 (a prime number) greater than 2.
So, x = y is not always true.

(II) y = 1 is again not always true.

(III) x and y are prime integers is again not true because if we take x = 10, y = 6, then 3x/y = 5
So, again it is not true always.

The correct answer is A.

Last edited by Anurag@Gurome on Fri Dec 16, 2011 3:30 am, edited 1 time in total.

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karthikpandian19: Legendary Member; Posts: 1665; Joined: Thu Nov 03, 2011 7:04 pmFollowed by: 70 members; Thanked: 165 times

by karthikpandian19 » Thu Dec 15, 2011 10:35 pm

In your proving portion, you had considered "I" y=3, but as per the statement y"not equal to" 3?????

Anurag@Gurome wrote:
karthikpandian19 wrote:If y â‰ 3 and 3x/y is a prime integer greater than 2, which of the following must be true?
â… . x = y
â…¡. y = 1
â…¢. x and y are prime integers.

(A) None
(B) â… only
(C) â…¡only
(D) â…¢only
(E) â… and â…¢
(I) x = y
If x = 5, y = 5, then 3x/y = 3 (a prime) greater than 2.
If x = 7, y = 3, then 3x/y = 7 (a prime number) greater than 2.
So, x = y is not always true.

(II) y = 1 is again not always true.

(III) x and y are prime integers is again not true because if we take x = 10, y = 6, then 3x/y = 5
So, again it is not true always.

The correct answer is A.

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ankush123251: Senior | Next Rank: 100 Posts; Posts: 49; Joined: Thu Feb 17, 2011 1:36 amFollowed by: 2 members; Thanked: 8 times

by ankush123251 » Fri Dec 16, 2011 3:06 am

@ Anurag@Gurome:
Correct me if i am wrong:In your proof firstly you took y = 3 but in question its clearly given y is not 3.
Secondly you took it under x=y,and x=7,y=3?? if x= 7 and x= y how can y be 3.

I believe the answer is A.
Since x = y always results in 3x/y being 3 which is greater than 2 and prime integer.
B)y =1 leads to 3x which depends on value of x,hence not true.
C)X and Y being prime integers leads to 3x/y not being prime integer.
Thus A.

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Fri Dec 16, 2011 3:31 am

ankush123251 wrote:@ Anurag@Gurome:
Correct me if i am wrong:In your proof firstly you took y = 3 but in question its clearly given y is not 3.
Secondly you took it under x=y,and x=7,y=3?? if x= 7 and x= y how can y be 3.

I believe the answer is A.
Since x = y always results in 3x/y being 3 which is greater than 2 and prime integer.
B)y =1 leads to 3x which depends on value of x,hence not true.
C)X and Y being prime integers leads to 3x/y not being prime integer.
Thus A.

My mistake, i have edited the previous reply. Thanks for pointing it out.

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ankush123251: Senior | Next Rank: 100 Posts; Posts: 49; Joined: Thu Feb 17, 2011 1:36 amFollowed by: 2 members; Thanked: 8 times

by ankush123251 » Fri Dec 16, 2011 3:38 am

Is the answer None?

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Fri Dec 16, 2011 3:43 am

ankush123251 wrote:Is the answer None?

Yes, that's right, it's none.

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